1. Field
This disclosure relates generally to pigment clusters made from a binder, opaque polymer, pigment, and small-size extender. The pigment clusters may be used in a paint product or paint composition in place of at least a portion of large-size extender particles to improve the opacity of resultant films made using the paint product while maintaining the mechanical integrity and flatness of the resultant film relative to a film formed from a paint product that lacks the pigment clusters.
2. Description of Related Art
Professional and residential consumers of surface coatings, such as architectural paints, prefer paint products that require the fewest number of applications of paint to cover the surface underneath the paint being applied.
Certain architectural paints are required by the professional or residential customer to have a low gloss or flat finish. To achieve such low gloss or flat finishes, manufacturers often add large particle size extenders which may be ground up minerals (such as calcium carbonate) with an average particle size from about 5 μm to about 50 μm. Such large size extenders are preferred by manufacturers due to their relatively low cost and high efficacy at increasing the pigment volume concentration of a coating. However, when added to a paint product, the large size extenders cause TiO2, which is nearly universally used in paint as a white opacifying pigment, or other pigment to crowd and such crowding decreases the efficiency of light scattering within the paint when it is applied as a surface coating. The large size extenders may also cause a reduction in opacity themselves by creating regions of low light scattering within the paint when it is applied as a surface coating. The consequence of having regions of low light scattering in a paint film is the appearance of microscopic inhomogenieties in the paint film. In other words, a coated surface will appear to have “spots” or “windows” on it.
To reduce the negative perception of the appearance of transparent windows that occur in matte paints which are caused by the presence of these large size extenders, manufacturers may increase the amount of TiO2 in the paint composition. However, this is not a preferred solution in part because of the high cost of TiO2.
The use of clay particles which may have high refraction index oxides or TiO2 mechanically attached to them is also commonly used in paint products as alternative large-size extenders. For example, in U.S. Pat. No. 3,726,700 to Wildt, the use of a composite pigment with clay particles having TiO2 and other materials adhered to the clay particles as a pigment that has high hiding power when used in a paint is described. U.S. Pat. No. 5,551,975 to Freeman et al. describes pigments for paints which are the result of the reaction of clay with colloidal silica. While such clay-based pigments are important advancements as alternatives to large size extenders for the use in surface coatings, such clay-based pigments do not effectively increase the opacity of a paint film because clay-based pigments tend to uptake the other materials within the paint (e.g., solvent, latex, etc.) due to the large sized pores and high porosity of the surface of the clay and, as such, the composite particles tend to lose their void volume, which is a key requirement for increasing opacity in a paint film.
The use of vesiculated beads to increase the light scattering (or opacity) of a paint is described in U.S. Pat. No. 4,489,174 to Karickhoff. Such vesiculated beads may increase opacity by up to 15% at like TiO2 concentrations when compared to a paint that has been traditionally formulated with only large size extenders. Such vesiculated beads are a continuous polymeric matrix that help improve the opacity and sheen qualities of a surface coating. The limitations of such vesiculated beads are that the beads tend to be costly due in part to the high concentration of polymer in the bead and are difficult to synthesize with uniformity which causes inconsistent light scattering efficiency and tint strength.
The use of larger pigment particles with a polymer that is covalently bonded to the pigment is described in U.S. Pat. No. 7,081,488 to Bardman et al. These polymer pigment particles purport to improve light scattering qualities of a paint coating. However, there is not any void space in such pigment particles and the cost of such pigment particles is relatively high.